Climate change dialogues

Nature Geoscience 5, 301 (2012). doi:10.1038/ngeo1474

Human influence on the planet is undeniable. Making a switch from exploitation to maintenance of natural resources depends on a step change in communication, to convince the Earth's population of the necessity for a fundamental change of course.

Nature Geoscience 5, 305 (2012). doi:10.1038/ngeo1456

Author: Graham Cogley

The fate of glaciers in the greater Himalaya is widely discussed, but poorly known. A new measurement in the central Karakoram mountain range suggests that glacier mass change in this region contributes to sea-level rise nearly 0.05 mm per year less than has been thought.

Nature Geoscience 5, 306 (2012). doi:10.1038/ngeo1457

Author: Ross Secord

The hothouse climate of the early Eocene epoch was punctuated by a series of transient warming events linked to massive carbon release. Detailed terrestrial records for three of these events indicate that they were caused by similar underlying mechanisms.

Nature Geoscience 5, 307 (2012). doi:10.1038/ngeo1463

Author: Jochen Braunmiller

Faults break under the stress of plate tectonic forces, but the processes immediately preceding rupture are enigmatic. Monitoring of a remote oceanic fault that breaks regularly indicates that rupture is controlled by physical properties of the fault zone.

Nature Geoscience 5, 310 (2012). doi:10.1038/ngeo1461

Author: Sinéad Collins

The prediction of marine microbial responses to ocean acidification is a key challenge for marine biologists. Experimental evolution offers a powerful tool for understanding the forces that will shape tomorrow's microbial communities under global change.

Nature Geoscience 5, 313 (2012). doi:10.1038/ngeo1438

Authors: T. Woollings, J. M. Gregory, J. G. Pinto, M. Reyers & D. J. Brayshaw

A poleward shift of the mid-latitude storm tracks in response to anthropogenic greenhouse-gas forcing has been diagnosed in climate model simulations. Explanations of this effect have focused on atmospheric dynamics. However, in contrast to storm tracks in other regions, the North Atlantic storm track responds by strengthening and extending farther east, in particular on its southern flank. These adjustments are associated with an intensification and extension of the eddy-driven jet towards western Europe and are expected to have considerable societal impacts related to a rise in storminess in Europe. Here, we apply a regression analysis to an ensemble of coupled climate model simulations to show that the coupling between ocean and atmosphere shapes the distinct storm-track response to greenhouse-gas forcing in the North Atlantic region. In the ensemble of simulations we analyse, at least half of the differences between the storm-track responses of different models are associated with uncertainties in ocean circulation changes. We compare the fully coupled simulations with both the associated slab model simulations and an ocean-forced experiment with one climate model to establish causality. We conclude that uncertainties in the response of the North Atlantic storm track to anthropogenic emissions could be reduced through tighter constraints on the future ocean circulation.

Nature Geoscience 5, 318 (2012). doi:10.1038/ngeo1452

Authors: E. A. Kort, S. C. Wofsy, B. C. Daube, M. Diao, J. W. Elkins, R. S. Gao, E. J. Hintsa, D. F. Hurst, R. Jimenez, F. L. Moore, J. R. Spackman & M. A. Zondlo

Uncertainty in the future atmospheric burden of methane, a potent greenhouse gas, represents an important challenge to the development of realistic climate projections. The Arctic is home to large reservoirs of methane, in the form of permafrost soils and methane hydrates, which are vulnerable to destabilization in a warming climate. Furthermore, methane is produced in the surface ocean and the surface waters of the Arctic Ocean are supersaturated with respect to methane. However, the fate of this oceanic methane is uncertain. Here, we use airborne observations of methane to assess methane efflux from the remote Arctic Ocean, up to latitudes of 82° north. We report layers of increased methane concentrations near the surface ocean, with little or no enhancement in carbon monoxide levels, indicative of a non-combustion source. We further show that high methane concentrations are restricted to areas over open leads and regions with fractional sea-ice cover. Based on the observed gradients in methane concentration, we estimate that sea–air fluxes amount to around 2 mg d−1 m−2, comparable to emissions seen on the Siberian shelf. We suggest that the surface waters of the Arctic Ocean represent a potentially important source of methane, which could prove sensitive to changes in sea-ice cover.

Nature Geoscience 5, 322 (2012). doi:10.1038/ngeo1450

Authors: Julie Gardelle, Etienne Berthier & Yves Arnaud

Assessments of the state of health of Hindu-Kush–Karakoram–Himalaya glaciers and their contribution to regional hydrology and global sea-level rise suffer from a severe lack of observations. The globally averaged mass balance of glaciers and ice caps is negative. An anomalous gain of mass has been suggested for the Karakoram glaciers, but was not confirmed by recent estimates of mass balance. Furthermore, numerous glacier surges in the region that lead to changes in glacier length and velocity complicate the interpretation of the available observations. Here, we calculate the regional mass balance of glaciers in the central Karakoram between 1999 and 2008, based on the difference between two digital elevation models. We find a highly heterogeneous spatial pattern of changes in glacier elevation, which shows that ice thinning and ablation at high rates can occur on debris-covered glacier tongues. The regional mass balance is just positive at +0.11±0.22 m yr−1 water equivalent and in agreement with the observed reduction of river runoff that originates in this area. Our measurements confirm an anomalous mass balance in the Karakoram region and indicate that the contribution of Karakoram glaciers to sea-level rise was −0.01 mm yr−1 for the period from 1999 to 2008, 0.05 mm yr−1 lower than suggested before.

Nature Geoscience 5, 326 (2012). doi:10.1038/ngeo1427

Authors: Hemmo A. Abels, William C. Clyde, Philip D. Gingerich, Frederik J. Hilgen, Henry C. Fricke, Gabriel J. Bowen & Lucas J. Lourens

Pronounced transient global warming events between 60 and 50 million years ago have been linked to rapid injection of isotopically-light carbon to the ocean–atmosphere system. It is, however, unclear whether the largest of the hyperthermals, the Palaeocene–Eocene Thermal Maximum (PETM; ref. ), had a similar origin as the subsequent greenhouse climate events, such as the Eocene Thermal Maximum 2 and H2 events. The timing and evolution of these events is well documented in marine records, but is not well constrained on land. Here we report carbon isotope records from palaeosol carbonate nodules from the Bighorn Basin, Wyoming, USA that record the hyperthermals. Our age model is derived from cyclostratigraphy, and shows a similar structure of events in the terrestrial and marine records. Moreover, the magnitude of the terrestrial isotope excursions is consistently scaled with the marine records, suggesting that the severity of local palaeoenvironmetal change during each event was proportional to the size of the global carbon isotope excursion. We interpret this consistency as an indication of similar mechanisms of carbon release during all three hyperthermals. However, unlike during the PETM (refs , ), terrestrial environmental change during the subsequent hyperthermals is not linked to substantial turnover of mammalian fauna in the Bighorn Basin.

Nature Geoscience 5, 330 (2012). doi:10.1038/ngeo1449

Authors: Florian Le Pape, Alan G. Jones, Jan Vozar & Wei Wenbo

The weak lithosphere of the Tibetan plateau is surrounded by rigid crustal blocks and the transition between these regimes plays a key role in the ongoing collision between India and Eurasia. Geophysical data and magmatic evidence support the notion that partial melt exists within the anomalously hot crust of northern Tibet. The Kunlun Fault, which accommodates the plateau’s eastward extrusion, has been identified as a significant rheological boundary between weak, warm Tibetan crust and the rigid eastern Kunlun–Qaidam block. Here we present reanalyses and remodelling of existing magnetotelluric data, using an anisotropy code to obtain revised resistivity models. We find unequivocal evidence for anisotropy in conductivity at the northern edge of the Tibetan plateau. We interpret this anisotropy as the signature of intrusion of melt that penetrates north from the Tibetan plateau and weakens the crust beneath the Kunlun Shan. We suggest that our identification of a melt intrusion at the northern edge of the Tibetan plateau compromises the previous identification of the Kunlun Fault as an important rheological boundary. We conclude that the crustal melt penetration probably characterizes the growth of the plateau to the north, as well as accommodating the north–south crustal shortening in Tibet.

Nature Geoscience 5, 336 (2012). doi:10.1038/ngeo1454

Authors: Jeffrey J. McGuire, John A. Collins, Pierre Gouédard, Emily Roland, Dan Lizarralde, Margaret S. Boettcher, Mark D. Behn & Robert D. van der Hilst

On a global scale, seismicity on oceanic transform faults that link mid-ocean ridge segments is thermally controlled. However, temperature cannot be the only control because the largest earthquakes on oceanic transform faults rupture only a small fraction of the area that thermal models predict to be capable of rupture. Instead, most slip occurs without producing large earthquakes. When large earthquakes do occur, they often repeat quasiperiodically. Moreover, oceanic transform faults produce an order of magnitude more foreshocks than continental strike-slip faults. Here we analyse a swarm of about 20,000 foreshocks, recorded on an array of ocean-bottom seismometers, which occurred before a magnitude 6.0 earthquake on the Gofar transform fault, East Pacific Rise. We find that the week-long foreshock sequence was confined to a 10-km-long region that subsequently acted as a barrier to rupture during the mainshock. The foreshock zone is associated with a high porosity and undergoes a 3% decrease in average shear-wave speed during the week preceding the mainshock. We conclude that the material properties of fault segments capable of rupturing in large earthquakes differ from those of barrier regions, possibly as a result of enhanced fluid circulation within the latter. We suggest that along-strike variations in fault zone material properties can help explain the abundance of foreshocks and the relative lack of large earthquakes that occur on mid-ocean ridge transform faults.

Nature Geoscience 5, 342 (2012). doi:10.1038/ngeo1447

Authors: E. Contreras-Reyes, J. Jara, I. Grevemeyer, S. Ruiz & D. Carrizo

No large tsunamigenic earthquake has occurred in north Chile since 1877 and the region has been largely recognized as a mature seismic gap. At the southern end of the seismic gap, the 2007 Mw 7.7 Tocopilla earthquake ruptured the deeper seismogenic interface, whereas the coupled upper interface remained unbroken. Seismological studies onshore show a gently varying dip of 20° to 30° of the downgoing Nazca plate, which extends from the trench down to depths of 40–50 km. Here, we study the lithospheric structure of the subduction zone of north Chile at about 22° S, using wide-angle seismic refraction and reflection data from land and sea, complemented by hypocentre data recorded during the 2007 Tocopilla aftershocks. Our data document an abrupt increase in the dip of the subducting plate, from less than 10° to about 22°, at a depth of approximately 20 km. The distribution of the 2007 aftershocks indicates that the change in dip acted as a barrier for the propagation of the 2007 earthquake towards the trench, which, in turn, indicates that the subduction megathrust is not only segmented along the trench, but also in the direction of the dip. We propose that large-magnitude tsunamigenic earthquakes must cross the barrier and rupture the entire seismogenic zone.

Nature Geoscience 5, 346 (2012). doi:10.1038/ngeo1441

Authors: Kai T. Lohbeck, Ulf Riebesell & Thorsten B. H. Reusch

Nature Geoscience 5, 352 (2012). doi:10.1038/ngeo1451

Authors: Niko Kampman, Neil M. Burnside, Zoe K. Shipton, Hazel J. Chapman, Joe A. Nicholl, Rob M. Ellam & Mike J. Bickle

Nature Geoscience 5, 359 (2012). doi:10.1038/ngeo1425

Authors: Aubrey L. Zerkle, Mark W. Claire, Shawn D. Domagal-Goldman, James Farquhar & Simon W. Poulton